In recent years, the reduction in size, weight, and thickness of electronic appliances such as notebook personal computers and mobile phones has proceeded. Therefore, higher energy density and reduction in weight are required also for secondary batteries for electronic appliances, and development of lithium ion batteries having high energy density has been actively made instead of conventional lead storage batteries and nickel hydride batteries. Further, a lithium ion battery which can be used also as a power source of an electric vehicle or a hybrid car has been put in practical use.
In the lithium ion battery, a compound containing lithium is used as a positive electrode material, and a carbon material such as graphite and coke is used as a negative electrode material. Further, between a positive electrode and a negative electrode, there is provided an electrolytic solution in which a lithium salt such as LiPF6 and LiBF4 as an electrolyte is dissolved in an aprotic solvent having osmotic force such as ethylene carbonate, propylene carbonate and diethyl carbonate or an electrolyte layer comprising a polymer gel impregnated with the electrolytic solution.
Conventionally, as a packaging material for a battery case, there has been known a laminate in which a stretched heat resistant resin film layer as an outer layer, an aluminum foil layer, and a non-stretched thermoplastic resin film layer as an inner layer are laminated in this order. In the case of a battery case obtained by using a packaging material for battery cases having such a structure, if a solvent having osmotic force like an electrolytic solution passes through a film layer serving as a sealant in a laminate used for the outer packaging of the battery, the laminate strength between an aluminum foil layer and a resin film layer may be reduced to cause the leakage of the electrolytic solution. Therefore, there has been developed a packaging material for battery cases in which an aluminum foil layer and an inner layer are bonded through an adhesive layer containing a resin containing a functional group having reactivity with isocyanates such as an acid anhydride group, a carboxyl group, and a hydroxy group, and a polyfunctional isocyanate compound.
For example, Patent Literatures 1 to 3 describe a method involving forming an adhesive layer using a solvent type adhesive in which a modified polyolefin resin obtained by graft-polymerizing an ethylenically unsaturated carboxylic acid or an anhydride thereof onto a propylene homopolymer or a copolymer of propylene and ethylene, and a polyfunctional isocyanate compound, are dissolved or dispersed in an organic solvent.
However, the modified polyolefin resin in Patent Literatures 1 to 3 was strongly influenced by factors determining the water content contained in the adhesive layer such as humidity at the time of coating and aging and the thickness of a substrate for a laminate. As a result, there was a problem that the adhesive layer formed showed variation in adhesive strength and was poor in electrolytic solution resistance.